The present invention relates to a code modulation (channel coding) system used for recording, reproducing and transmitting digitalized video signals.
Recently, units for digitalizing video signals (television signals) and recording/reproducing the signals have been vigorously developed. This is because the picture quality can be significantly improved in such devices as compared with the conventional analog recording system.
One of the representative units for recording/reproducing such digitalized television signals is a digital VTR.
The NRZ (Non Return to Zero) system is one of the available modulation systems for recording/reproducing the video data onto/from such a digital VTR. In the NRZ system, input data "1" and "0" are made to be associated with the high level and the low level of the signal amplitude, respectively.
In the NRZ system, however, the DC component and the low frequency components included in the signal increase when the input data "1" or "0" appears consecutively or when one of the data "1" and "0" appears more frequently than the other. If such a signal is transmitted via a rotary transformer as in the VTR, the low frequency components are cut off. As a result, the waveform is significantly deteriorated by the sag in the signal level and a number of code errors are produced.
One of the modulation systems for eliminating the above described drawbacks of the NRZ system is the 8-8 conversion system. Reference may be made to "RECENT DEVELOPMENTS FOR DIGITAL VTR: CHANNEL CODING AND ERROR PROTECTION" by J. Heitmann et al, International Broadcasting Convention IEEE 220, pp. 221-225, 1982. In this system, sufficiently high correlation between video signals is utilized to eliminate the DC component and the low frequency components included in the video data. However, the 8-8 conversion system is not completely satisfactory in that the DC and low frequency components are not eliminated sufficiently.